1. Field of the Invention
The present invention relates to a plunger rod for an injection device, said plunger rod allowing the delivery of selected doses of product while being very compact. The invention further relates to a compact injection device comprising such a plunger rod.
2. Description of Related Art
In this application, the distal end of an element or of a device means the end furthest away from the hand of the user and the proximal end means the end closest to the hand of the user, when the element or device is in the use-position. Similarly, in this application, the terms “in the distal direction” and “distally” mean in the direction of the injection, and the terms “in the proximal direction” and “proximally” mean in the direction opposite to the direction of injection.
Injection devices, such as syringes, are well known. Many different types of injection devices have been designed for administering medicines. Injection devices usually comprise a container intended to receive the product to be injected and a plunger rod intended to move a stopper within the container so as to expel the product therefrom at the time of injection. Empty and pre-filled disposable injection devices exist but prefilled devices are now preferred because they are more convenient, safe and efficient and may be used directly in emergency cases.
In some cases, the administration of a medicine requires the injection of a very specific dose of product. In other cases, the injection step may be divided into several successive steps, as for example where several doses of the product are to be injected one after the other. In addition, the successive doses of product to be injected may have different volumes.
The administration of certain particular medicinal products also may require a succession of precise steps. Thus, in the case of anti-cancer products for example, the active principle must be stored separately from the solvent with which it should only be mixed just before the time of injection. Injection devices such as syringes provided with two separate chambers, one containing the active principle and the other its solvent, are used for this purpose. In such cases, the plunger rod is displaced along a first distance, whereupon the active principle is transferred along a by-pass system and enters in the chamber containing the solvent. Once the medicine is reconstituted, the plunger rod is displaced along a second distance for completing the injection step as such. This step may itself be divided into several steps to inject several doses as already described.
In all the cases mentioned above, it is particularly important that the plunger rod be stopped exactly and precisely right after being displaced along a certain distance, and for a repeated number of times if necessary.
Moreover, prefilled injections devices are usually filled by a pharmaceutical company, packaged for use, and then typically stored at a doctor's office, hospital, etc. until they are needed for use. In such a condition, the prefilled injection device will take up a predetermined amount of storage space based upon the size of the injection device (typically including a syringe barrel, a stopper, a plunger rod, and possibly a needle). In some cases, the predetermined amount of storage space the injection device will take up is a maximum space approximating the length of the plunger rod, plus the length of the syringe barrel, plus the length of the needle (if provided).
Required storage space is an important feature for prefilled injection devices. It is especially important when the medicine contained in these devices must be stored and transported at low temperatures. Storage of these injection devices may require refrigeration and can be expensive. This is especially the case in hospitals and pharmacies, where storage space for medicines is limited.
Thus, there is a need for an injection device which would be particularly compact, especially when the injection device is prefilled, and that would ensure the possibility of delivering specific selected doses of product, in a successive manner or not. There is therefore still a need for a safe and compact injection device which would avoid at least temporarily any inadvertent movement of the plunger rod past a predetermined limit, this injection device nevertheless allowing a subsequent movement of said plunger rod for at least one or more subsequent steps, and in particular for the delivery of one or several additional selected doses of product.
Moreover, such an injection device must be simple to use, and preferably would not alter the typical process followed by a medical staff when administering a medicine by injection.